1 Technical Field
This invention relates to the field of materials requiring strength in high temperature environments. More particularly it relates to a material composed primarily of silicon nitride, also containing a rare earth metal oxide, but having no more than 0.5% alumina. This material is particularly suited for use as components of turbines and engines which are exposed to combustion temperatures.
2. Technical Background
Polycrystalline silicon nitride ceramics are a well known class of materials. They are commonly made by compressing either silicon or silicon nitride powder to give a coherent green body in the general shape of the final ceramic article desired. Depending on the method used for forming the green body, a fugitive binder may or may not be needed to give coherence to the green body, and a second compression step may or may not be advantageous. After adequate compression, the body is debinderized if necessary and then is finally converted into a form ready to use by a process called densification. If the body before densification consists primarily of elemental silicon, it may be converted to silicon nitride by exposure to nitrogen gas at an appropriate temperature, a process known as reaction bonding. If the body before densification is already primarily silicon nitride, densification is usually accomplished by a combination of heat and pressure.
Most metals and their oxides have lower melting points and are considerably weaker at high temperatures than silicon nitride. However, it has been found in practice that the presence of some lower melting component, called a densification or sintering aid, is necessary to allow densification of silicon nitride bodies under practically attainable conditions of temperature and pressure. The amount of sintering aid must be controlled carefully, because too much will weaken the product and too little will lead to inadequate densification. Some metals and a wide variety of oxides and mixtures of oxides, including yttria and other oxides of the rare earth metals, have been reported by others to be suitable densifying aids for silicon nitride to be used at high temperatures.
One of the most effective densification techniques is that generally known in the art as hot isostatic pressing (often abbreviated hereinafter as "HIP"). The technique of HIP best suited to manufacture of silicon nitride articles is that described in U.S. Pat. No. 4,339,271 of July 13, 1982 to Isaksson et al. Additional variations and improvements of this process, some of them particularly applicable to silicon nitride, are described in U.S. Pat. Nos. 4,081,272 of Mar. 28, 1978; 4,112,143 of Sep. 5, 1978; 4,256,688 of Mar. 17, 1981; 4,446,100 of May 1, 1984; and 4,455,275 of June 19, 1984; all to Adlerborn, either alone or with various coworkers. All these patents teach that a silicon nitride body should be degassed at a temperature of about 950.degree. C. before being encapsulated in the glass envelope in which HIP actually occurs.
U.S. Pat. No. 4,457,958 of July 3, 1984 to Lange et al. teaches the use of diffusion techniques after densification of silicon nitride bodies to improve the creep resistance and strength by reducing the amount of intergranular phase. While this technique is not at all closely related to that of the present invention, it did achieve a reported value of 82,000 psi or 565 MPa at 1400.degree. C. for the flexural strength of silicon nitride, one of the higher value known to the applicant from the prior art. The type of silicon nitride with which this high value for flexural strength was achieved contained deliberately added magnesia and almost certainly some silica as its primary densifying additive; it did not contain any significant amount of rare earth oxide. Although the technique taught by Lange was applied to some silicon nitride bodies which did have yttria as the primary glass forming densification aid, the flexural strength values for these samples were not reported; only improvements in creep strength were reported for these yttria-containing samples.
D. C. Larsen et al., Ceramic Materials for Advanced Heat Engines (1985), reviews the effect of various densifying aids on the high temperature properties of silicon nitride. This reference reports one material, containing 4% yttria and 3% alumina, which achieved flexural strengths of as much as 100,000 psi or nearly 700 MPa at about 1370 C (see graphs on pages 121 and 127.) However, it is also noted that this material "appears to be oxidation limited at 1500 C. This is thought to be due to the Al.sub.2 O.sub.3 additive." (page 120). It is also believed by the present applicant that the use of alumina as a densifying aid in silicon nitride is likely to result in relatively poorer high temperature strength at low strain rates than at high strain rates, when compared with silicon nitride containing rare earth oxides such as yttria, substantially free from alumina, as the densifying aid.
The Larsen reference also notes (pages 120-24), "The success of Y.sub.2 O.sub.3 as a densification aid for HP-Si.sub.3 N.sub.4 lies in the fact that the resulting yttrium silicate intergranular phase can be crystallized. If more than 4% Y.sub.2 O.sub.3 is used (i.e., 8% or more), we have found that there is a strong tendency to be in that part of the Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 -SiO.sub.2 phase triangle that results in oxynitride phases that are unstable in oxidizing environments." In a later passage (page 221), the same reference notes that Si.sub.3 Y.sub.2 O.sub.3 N.sub.4, YSiO.sub.2 N, and Y.sub.10 Si.sub.7 O.sub.23 N.sub.4 phases are not desirable intergranular constituents because they are susceptible to rapid oxidation, which can lead to catastrophic failure of the silicon nitride bodies with such intergranular phases. However, an intergranular phases of Y.sub.2 Si.sub.2 O.sub.7 is recommended as free from this difficulty.
Japanese Patent Application No. 56-185122 of Nov. 17, 1981, published May 26, 1983 under No. 58-88171, describes a method of preparing dense silicon nitride bodies by preparing green bodies, heating them in a nitrogen atmosphere, and then finally densifying the bodies by HIP. However, the heating recommended by this reference is at temperatures above 1600 C and the microstructural effect intended to be accomplished by the heating is transformation of the crystal form of the silicon nitride from alpha to beta. Flexural strengths for the products made according to this reference are given only at room temperature and 1200 C. No indication of the units intended for the flexural strength values could be found, but it is likely that units of kg/mm.sup.2 were intended. The highest value reported at 1200 C is 74.